Synthetic aperture radar (SAR) exploits the motion of an airborne antenna in order to simulate a large antenna electronically. In conventional radar systems, the resolution of the resulting image increases with the size of the antenna. An advantage of SAR is the ability to achieve high image resolutions with a comparatively small antenna.
In SAR, the radar measures distances in slant-range by observing the time taken for an emitted signal to be reflected back to the radar from an object or the ground. As the distance to objects is measured in the slant-range, rather than the true horizontal distance along the ground, the resulting images are subject to geometric distortions. Therefore, SAR images require image processing in order to accurately reflect the appearance of “objects” (including terrain features, buildings, and any other objects of interest).
One such geometric distortion is the effect of layover. Layover occurs when the radar beam is reflected from the top of an object before it is reflected from the bottom of the object. The top of the object therefore appears closer to the radar than the bottom of the object, and when viewed on the resulting image, the top of the object “lays over” the bottom of the object.
Another feature of the resulting images from SAR is the effect of shadow. Shadows appear in the resulting images when the radar beam is unable to illuminate a particular area owing to the presence of an object interposed between the radar emitter and the area immediately beyond the object in the slant-plane. The electromagnetic radiation is reflected from the object itself and is therefore unable to penetrate the region beyond the object in the slant-plane.
Owing to the nature and geometry of SAR images, recognition of objects in SAR imagery is more challenging than in optical imagery. It is therefore desirable to process images efficiently in order to simplify the recognition and classification of objects. Current SAR imagery requires experienced image analysts to perform the task of recognition and classification.
Existing technology for processing SAR imagery presents the resulting images in either the slant-plane (the plane inclined at the angle between the line of sight from the radar to the object, and the ground), or the ground-plane. Often the resulting images processed by existing methods are presented using square pixels of consistent size. However, it can be difficult to accurately recognise objects present in the resulting images.